(function(doc,win){
    // 取页面的跟结点
    let docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in win ? 'orientationchange' : 'resize',
        recalc = function(){
            let clientWidth = docEl.clientWidth
            if(!clientWidth){
                return
            }
            docEl.style.fontSize = 20 * (clientWidth / 320) + 'px'
        } 
        if(!doc.addEventListener) return
        win.addEventListener(resizeEvt,recalc,false)
        doc.addEventListener('DOMContentLoaded',recalc,false)

})(document,window)